Humic acid (HA) was extracted from surface soil under evergreen broadleaf forest in Jingyun Mountains of China. Effects of acidity, ion strength and temperature on stability and thermodynamics of the complex reaction between the humic acid and Mn2+ ion were studied using the ion-exchange equilibrium method. Results showed that its stability constant increased with increasing pH when ion strength and temperature remained unchanged, and with increasing ion strength,as well. Ligancy of the complex reaction increased with pH rising from 3.0 to 5.0, but decreased with pH rising further up from 5.0 and 7.0, and increased as well with ion strength rising from 0.00 to 0.10 mol L^-1, but decreased, too, with ion strength rising further from 0.10 to 0.15 mol L^-1. The thermodynamic parameters of the complex reaction, including standard free energy change ΔGmθ, standard enthalpy change ΔHmθ, and entropy change ΔSmθ, were all negative when the temperature fell within the range from 298.2 to 308.2K, suggesting that the complex reaction is a kind of exothermic one processing spontaneously and keeping it lower in temperature favors process of the reaction.